For many patients with severe to profound hearing impairment, there are several types of middle and inner ear implants that can restore a sense of partial or full hearing. For example, cochlear implants can restore some sense of hearing by direct electrical stimulation of the neural tissue of the cochlea. The cochlear implant typically includes an electrode carrier having an electrode lead and an electrode array, which is threaded into the cochlea. The electrode array usually includes multiple electrodes on its surface that electrically stimulate auditory nerve tissue with small currents delivered by the electrodes distributed along the electrode array. These electrodes are typically located toward the end of the electrode carrier and are in electrical communication with an electronics module that produces an electrical stimulation signal for the implanted electrodes to stimulate the cochlea.
One of the important steps in cochlear implant surgery is the insertion of the electrode array into the scala tympani of the cochlea. However, the cochlear duct length, i.e., the length measured from the round window entrance until the most apex of the cochlea (helicotrema), varies from patient to patient so no single electrode array length will be suitable for all patients. Literature shows that the cochlear duct length typically varies from about 25 mm to 35 mm. Thus, manufacturers of cochlear implants provide a wide range of electrode array lengths so as to cover the diversified cochlear duct length in patients. Some cochlear electrodes include a fixed insertion stopper to indicate the maximum insertion depth for the electrode array. The insertion stopper may also serve as a cap to close the round window opening, or cochleostomy site, where the electrode array is inserted into the cochlea.
In some cases, however, during the electrode insertion process, the electrode array is not inserted into the scala tympani beyond a certain insertion depth and stops well before the fixed insertion stopper reaches the cochleostomy due to a variety of situations, e.g., the electrode array length is too long for that particular cochlea, greater anatomical resistance inside the cochlea, insufficient experience with the surgeon, etc. In such situations, the closing of the round window opening or cochleostomy by the fixed insertion stopper is not possible and the exact electrode array entry point into the scala tympani cannot be determined or identified in radiographs that are taken after implantation.